1. Field of the Invention
The present invention relates to an UV detector for liquid chromatography, and in particular relates to an improved flow cell structure.
2. Description of the Related Art
FIG. 1 shows an example of a prior art multiple wavelength detector used in liquid chromatography, in which light emitted from a light source 1 is converged by a converging system (represented by one lens in FIG. 1) 2 and shines into a flow cell 3. As shown in FIG. 2, the flow cell 3 is constructed from a cylindrical sample chamber 6 sandwiched between window 4, 4 made of a transparent material such as quartz or the like, and the light which shines into the flow cell 3 from the light source 1 enters the sample chamber 6 while a solvent flows therethrough. Then, as the light passes through the sample chamber 6, some of it is absorbed by the sample contained in the solvent.
The light which exits the flow cell 3 is dispersed by a light dispersing optical system (represented by one diffraction grating in FIG. 1) 7 and shines onto a photodiode array of a light detector 8 which converts each wavelength light intensity into electrical signals to obtain data about the light which passes through the flow cell 3.
In this connection, when the flow cell shown in FIG. 2 is used in a multiple wavelength detector for liquid chromatography, the following problem occurs.
Namely, the light shining into the flow cell has a poor utilization efficiency. This is due to the fact that the light from the light source 1 generally enter the flow cell at some solid angle. Accordingly, a portion of the light entering the flow cell 3 strikes the inner cylindrical walls and is absorbed, and because this portion of light does not pass through the flow cell 3, the utilization efficiency of the light entering the flow cell 3 is reduced. As a result, the detection signals (electrical signals) outputted from the light detector 8 (photodiode array) are weakened, and this causes an increase in the noise of the measured data.
One example proposal in the prior art for overcoming the problem described above is an invention disclosed in Japanese Laid-Open Patent Application No. HEI-8-3483. The solution proposed in HEI-8-3483 involves giving the sample chamber 6 a truncated cone shape which expands in the direction of travel of the light, as shown in FIG. 3. When the sample chamber 6 is given such a shape, the light which enters the flow cell 3 will not be absorbed because such light will not strike the inner wall surface of the sample chamber 6 even if the light enters at some solid angle. In this connection, Japanese Laid-Open Patent Application No. SHO-54-33871 and Japanese Laid-Open Utility Model Application No. SHO-40-14080 are publications which have a different object than that of the present invention but disclose a sample chamber having a truncated cone shape which expands in the direction of travel of the light like that described above.
However, if the sample chamber 6 has a shape like that shown in FIG. 3, the following problem occurs. Namely, even when the light absorbance is the same, if the solvent is replaced with another solvent having a different index of refraction, the output of the light detector 8 will change, and this will cause a change in the base line of the measured data. The cause of such change is believed to be as follows. Namely, when a new solvent having an index of refraction different from that of the solvent flowing up to the current moment is flowed into the flow cell 3, the portion of the solvent roughly near the center of the sample chamber 6 has a fast flow rate, and this gives the portion of the new solvent in the center of the sample chamber 6 a higher density than that of the portion near the inner walls of the sample chamber 6. Now, because this creates an effect like that of a lens (known as the "lens effect"), the light entering the flow cell 3 is refracted, and a portion of such refracted light strikes the inner walls of the sample chamber 6 and is absorbed. As a result, the output of the light detector 8 will change, and this will cause the above-described change in the base line of the measured data.
Further, the spectrophotometer disclosed in Laid-Open Patent Application No. HEI-2-259452 is similar in structure to the present invention. Looking solely at the flow cell structure, the invention disclosed in the HEI-2-259452 has a sample chamber formed with an expanded inner shape at the light entrance side. However, in this published invention, a slit must be arranged between the flow cell and the light source, and the light forms an image near the slit at a position away from the light entrance side of the flow cell.
When such structure is used, the light emitted from the light source enters the flow cell after being constricted by the slit, and the portion of the light traveling to the outside is reflected by the inner surface of the sample chamber and does not reach the photosensor. In other words, the elements are arranged so that the photosensor receives only the portion of light that is roughly paraxial to a light path not affected for the most part by changes in the internal conditions of the flow cell, such as changing the solvent or the like. Consequently, there is a loss in sensitivity due to the reduction in the amount of light that is received and used for measurements.